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#868361 01/07/22 5:04 am
Joined: May 2009
Posts: 389
Britbike forum member
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Britbike forum member
Joined: May 2009
Posts: 389
The SU carb on my 1958 Square Four has a brass nozzle to the left of the intake but I’m unsure what it is for. It appears to look like a vacuum tube should be attached (?? To automatic advance in a car application). I’d like to know if that should be capped if not used or does it play a part in providing lift for the piston, please.
I have attempted to add a photo - hope I was successful.
Thanks in advance,


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Hi Paul,
No photos are showing up.
I've worked on a bunch of SU carbs on Jag's, MG's, Triumph cars, Aston Martins and more that I can't remember now.
That fitting as you suspect is most likely for a vacuum advance distributor on a car.
It should be capped if not connected to a distributor. If left open, It will interfere with proper response of the variable venturi (piston) and may contribute to a weak mixture at idle or a lean flat spot just off of idle..

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